subarrays. The effect of finite gaps between the subarrays upon grating lobe peaks is shown in Fig. 16. This degradation (increase in grating lobe peaks) occurs because Lx in Eq. 26. The result indicates a 15 cm gap as originally proposed was unacceptable; however, the 0.635 cm gap for the reference design has minimal impact. 7. CONCLUSION In summary, only .Y-axis and F-axis grating lobes with antenna tilts in those axial directions are important in determining antenna attitude control constraints and antenna flatness requirements for the SPS. Phase control to the power module level is quite beneficial in reducing the grating lobe peaks and/or tolerating greater errors in attitude control. Off-axis grating lobes are 20-40 dB lower than corresponding X and Y axis lobes. Antenna tilt requirements, or attitude control constraints, vary from one arc-minute for phase control at the subarray level to approximately 6 arc-min for power module phase control. Acknowledgements — The authors would like to acknowledge the informative discussions with W.B. Warren of TRW Systems, Houston, Texas, S. Rathjen and W. Lund of the Boeing Aerospace Company, Seattle, Washington, and R.H. Durrett of Marshall Space Flight Center, Huntsville, Alabama. REFERENCES 1. W.C. Lindsey and A.V. Kantak, Solar Power Satellite Baseline Phase Control System Design and Performance Evaluation, Phase 11 Final Report, NAS 9-15237. 2. S. Silver, Microwave Antenna Theory and Design, Radiation Laboratory Series, p. 173, 1949. 3. R. Courant and D. Hilbert, Methods of Mathematical Physics— Vol. I, Interscience, New York, 1953. 4. Department of Energy, SPS Reference System Report, DOE/ER-0023, 1978.
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